Lubricating materials have been used in the sliding parts and rotating parts of the various kinds of industrial machines, not least in the automobile industry. Very many of these machines use grease lubrication in order to simplify the seal structure and enable the apparatus to be small and compact.
The range of use of grease lubrication is extremely wide, for example, in the various types of rolling bearings and sliding bearings which support a rotating body, in sliding screws or ball screws having a feed screw structure, linear guides having a translation structure, ball joints having a link structure, and also in various kinds of gears.
As the requisite quality of industrial machines has improved year by year, the performance required has also reached a high level, and there are now many machines which aim for differentiation by adding various specifications.
In particular, the technical innovation in automobile electric power steering devices is remarkable, such that these devices, which were initially only used in some solar cars and light automobiles, are now very widely installed in small to medium-sized passenger cars. This is a vigorously growing sector wherein the number of such devices installed is almost doubling every year.
In electric power steering devices an electric motor is used as the power assist power source. By means of a control unit, it is possible to drive the electric motor only at times when the power assist is necessary. Moreover, since the electric motor drive uses electricity generated when the car is running, the engine power loss is very small. Accordingly, there is a substantial fuel economy effect, and energy consumption is decreased greatly compared to hydraulic power steering devices.
However, since the power output generated by current electric power steering devices is still low compared to that from hydraulic power steering devices, it is important not only to increase the electric motor power but also to decrease the load on the motor to the maximum extent by reducing friction among individual component parts as much as possible.
The improvement in quality and features of the above-mentioned machines is of course often in elements that correspond to design, but the operating conditions at the sliding parts and behaviour such as friction fluctuations are largely related to the lubricants used. The lubricant characteristics are also very important in respect of smooth handling conditions or consistent movements, and also the feedback sensed by equipment operators.
For example, in the case of a car's steering apparatus, the sensations felt by the driver while handling it are very important. If it feels too light, the driver will feel unsafe. If it is too heavy, handling will be detrimentally affected and it will the give the driver an uncomfortable feeling of effort. Moreover, the feeling when operating the steering must not be the same when driving straight ahead and when manoeuvring. If handling while driving straight ahead is possible with tiny movements, the consistent and gentle sensation of steering will contribute to safe forward progression of the car and will give a feeling of a satisfying drive where the driver is safe. If steering in reverse, operation must also give a light and stable feeling.
Furthermore, in order to finish a workpiece accurately and with good precision on the XY table of a machine tool, stable operating characteristics are extremely important. If frictional phenomena such as fluctuations or breaks in the oil film occur, these may lead to a reduction in the quality of the workpiece, and the accuracy of the precision of the machining will be lost.
Apart from these cases, there are the sliding parts of cooling fan bearings in cars and the various gears and bearings of the steering apparatus unit, the bearings of rack guides, ball joints and air compressors. Since these car parts frequently undergo repeated stop-start operation, they may be said to be in a lubricating environment where friction fluctuations are likely to occur. Bucket pins of construction machines such as power shovels and bulldozers, or the sliding parts of turning gears and crane booms also undergo repeated stop-start operation and are also in a lubricating environment where friction fluctuations are likely to occur.
Furthermore, table rollers in, for example, steelmaking equipment repeat the operation of rotating as the steel material passes through and stopping once the steel material has gone through. In the case of journal bearings in a forging press, the crank actuates the eccentric shaft only when the material is being processed. Since the workpiece is also subjected to pressing processes, the bearings used here may be said to be in an environment where friction and torque fluctuations are likely to occur because they are subjected to conditions of repeated stop-starts.
The factors under which these irregular friction fluctuations occur are in an environment of 100% relative sliding in, for example, the sliding screws of machine tools, the suspension ball joints of automobiles and the journal bearings of forging presses, where no rotating body is present. When supply or intervention of a grease is insufficient or the prescribed lubricant film is not formed, friction fluctuations are generated. These friction fluctuations are particularly likely to occur in the process of shifting from the stop state to the operating state.
Even though gear apparatuses have a different structure, sliding friction also constantly occurs at the contact points between the gears. Consequently, if the supply or intervention of a grease is insufficient and the viscoelasticity of the intervening grease is insufficient or reduced, friction fluctuations are generated and wear also increases.
Further, in the various types of rolling bearings, ball screws or the like in which rotating bodies are interposed, the distances of the raceway surface on which the rotating bodies are interposed differ in internal and external diameters. Thus, sliding occurs between the rotating bodies such as balls or rollers interposed there and the actuating surface. Also, in mechanisms typically represented by ball screws with no retainers present where a plurality of balls is disposed, the balls thus interposed rotate and come into contact with each other, so that relative sliding occurs on their contact surfaces. Also, in the process of moving from normal rotation to reverse rotation, differences in the spacing between balls occur, so that time gaps arise before the balls settle and revert to the rotating state. Whenever the oil film or the viscoelasticity of the grease is insufficient, conditions are generated under which friction fluctuations such as stick-slip will be likely to occur.
Therefore, to enhance machine reliability and safety, it is extremely important to reduce the friction fluctuations in the sliding parts of such machinery and so switch over to a stable rolling and/or sliding state.
Hitherto there have been many patent documents contributing to enhancement of friction properties and lubrication characteristics. However, virtually none of the documents disclose techniques for preventing friction fluctuations.
Japanese Laid-open Patent Application 1985-31598 discloses a technique in which the operating torque of suspension ball joints, for example in cars, is reduced by application of a ball-joint grease composition in which a paraffin wax or a fatty acid amide wax and a urea thickener are blended in a poly-α-olefin type synthetic oil having a viscosity at 40° C. of 500 to 2000 cSt. However, whilst the urea compound and fatty acid amide wax disclosed in said document resemble aspects of the grease composition of the present invention, the grease described therein differs from the lubricating grease composition of the present invention in respect of the three compounds which are combined as the thickener constituent. Furthermore, said document is concerned only with torque reduction. In contrast, the present invention offers an effect in which sporadically generated irregular friction fluctuations are reduced in the sliding parts of machines and stable friction characteristics are imparted. Hence, the present invention is concerned with completely different problem from said document.
Japanese Laid-open Patent Application 1990-194095 discloses a technique in which a ball-joint grease composition containing a urea-type thickener and a specified dehydrogenated dewaxed base oil and paraffin wax or a fatty acid amide wax. Said grease composition is said to give rise to small operating torque in a ball joint in an automobile or the like and also has no detrimental effect on the protective-boot rubber. However, whilst the urea compound and fatty acid amide wax described in said document resemble the grease composition of the present invention, the thickener constituents of the present invention and the problem addressed by the present invention differ completely.
Japanese Laid-open Patent Application 1996-209167 discloses a grease composition for resin lubrication comprising a thickener, a base oil and 1 to 10 wt % of at least one fatty acid containing a hydroxyl group or fatty acid ester of a polyhydric alcohol, based on the total weight of said grease composition. The use of said grease composition results in a sufficient thickness of an oil film secured in lubrication between a metal and a resin. In addition, when said grease composition is applied to power transmission mechanisms such as power steering apparatus, the generation of torque fluctuations is suppressed even over long periods of use. However, the grease composition of JP 1996-209167 A is different to the grease composition of the present invention.
Japanese Laid-open Patent Application 2002-265970 discloses a grease composition which is said to have excellent acoustic performance and anti-fretting properties. Said grease composition is characterised in that the thickener is formed from a mixture of a urea compound and a lithium soap in a grease composition wherein the main constituents are a base oil and a thickener. Whilst the urea compound and the lithium soap described in said document resemble part of the grease composition of the present invention, fundamentally, the constituent components of their thickeners are different and the problem addressed by the present invention differs completely.
Japanese Laid-open Patent Application 2004-083797 discloses a technique for a grease composition containing a base oil and a thickener in which the grease composition is characterised in that the thickener is constituted by a polyurea and a metallic soap. Said grease composition has excellent acoustic properties at low torque, as well as generating less dust even at high temperatures and being highly effective in rotating apparatus. However, whilst the urea compound and the metallic soap disclosed in said document resemble part of the grease composition of the present invention, fundamentally, the constituent components of their thickeners are different and the effect of the present invention differs completely.
Japanese Laid-open Patent Application 2004-301268 discloses an electric power steering apparatus which transmits an auxiliary output from an electric motor to the steering mechanism of a vehicle via a reduction gear mechanism, where the driven gears of the aforementioned reduction gear mechanism are comprised, as a whole, of the outer circumference of a metallic core pipe, a resin part which is comprised of a resin composition of which the gear teeth are formed, where said reduction gear mechanism is lubricated by means of a grease composition. Said grease composition uses a thickener such as a diurea compound containing a wax into which a group having a polarity has been introduced into the molecular structure, and where the sliding lubrication between the resin members and metal members which are the sliding parts of said reduction gear mechanism is maintained favourably over a long period with excellent steering feel. However, in the case of the grease composition of the present invention and the grease composition of said patent document, only the diurea compound and a part of the additive constituents simply resemble part of the grease composition of the present invention. Fundamentally, the grease compositions of JP 2004-301268 A and the present Application are different and the effect and configuration of the present invention differs completely.
Japanese Laid-open Patent Application 2004-314916 discloses an electric power steering apparatus which transmits an auxiliary output from an electric motor to the steering mechanism of a vehicle via a reduction gear mechanism. With regard to the aforementioned driven gears, there is also disclosed a grease containing a urea compound as the thickener and a lithium soap as an additive which is interposed between said gears. Said gears comprised, as a whole, of the outer circumference of a metallic core pipe and a resin composition of which the gears are formed. However, whilst the diurea compound and the metallic soap contained as an additive in said grease resemble part of the lubricating grease composition of the present invention, fundamentally, the grease compositions are different and the effect and configuration of the present invention differs completely.